This invention pertains generally to the field of electrolytic capacitors ("elcaps") and specifically to covers and the construction of covers for capacitors. It is contemplated that the invention will be particularly applicable to aluminum electrolytic capacitors.
Electrolytic capacitors, and specifically aluminum electrolytic capacitors, generate internal heat during operation because of fluctuating current ("ripple current") and internal resistance (Effective Series Resistance--"ESR") in accordance with the formula: Power (watts)=I.sup.2.sub.rs.times.ESR. The heat is generated internally in the active element of the roll and must diffuse outward to the packaging (or can) before it can be carried away by convection, conduction and/or radiation to the ambient environment. Radial and axial heat flows serve to conduct the heat from the core of the capacitor to the sides and bottom of a cylindrical package in which the capacitor may be encased. Construction details of the capacitor can facilitate or introduce resistance to these heat flows. Excessive internal heat can increase the electrolyte temperature and exacerbate corrosion problems.
The tabs and terminals of elcaps are at special risk of corroding since they are not in contact with the normal working electrolyte, but rather are in contact with the vapors and condensate of that electrolyte. The vapor or condensate is normally less corrosion-resistant because of higher levels of halide (especially chloride) contamination than the normal working electrolyte, preferential volatilization of solvent and less desirable electrolyte components. One likely source of chloride (and other contaminants) is the molded cover (or header or lid) through which electrical connection is made. If the electrolyte vapors and condensate extract any appreciable amounts of halides from the cover, it is mostly likely to migrate under the electric field to the terminals and tabs causing corrosion.
The cover material for aluminum electrolytic capacitors has traditionally been molded from either Nylon.RTM. or standard phenolic molding compositions, which contain porous fillers. These materials are satisfactory for use with older, and more corrosion inhibiting, electrolytes at low temperatures (.about.85.degree. C.), and for lifetimes up to 2,000 hours.
For electrolytic capacitors that are intended to operate at temperatures of greater than 85.degree. C., alternate solvents such as dimethylformamide (DMF) or butyrolactone can be chosen, since they have less tendency to corrode or react with the electrolytic capacitor components at these temperatures (as do the various glycol-based formulations that are commonly used for lower temperature applications). These alternate solvents, however, have undesirable properties such as carcinogenicity, teratogenicity, toxicity, high costs and a tendency to damage the cover, gaskets, etc. It is desirable to use ethylene glycol solvents and solve the corrosive tendencies associated with them for higher temperatures and longer times.
The development of longer life, higher temperature elcaps using environmentally acceptable ethylene glycol has been blocked, at least partly, by the lack of clean, durable elcap covers that are commercially available and inexpensive to manufacture as well as crack and craze. Polymeric molding compositions used in the commercial manufacture of elcap covers comprise thermoplastic or thermosetting resins (such as phenolic resins), fillers and other additives (such as pigments, lubricants and processing aids). Thermoplastics such as Nylon.RTM. and polypropylene are flammable and can soften at high temperatures as well as crack and craze. Thermoset materials (such as phenolic resins) are mechanically more stable for long term, high temperature applications. Phenolic resins are typically reaction products of phenols and aldehydes, such as formaldehyde. Fillers can be used up to about 50 volume percent of the molding compositions, and include wood flour, paper, cotton flock, minerals, chopped cloth, fibrous glass, etc. Such fillers can be classified as porous or nonporous.
Phenolic molding compositions can be divided into six general groups, although there is some overlapping. Briefly, these include (1) general-purpose (usually wood flour-filled); (2) impact (filled with cellulose, minerals, glass fibers); (3) nonbleeding (wood flour-filled with carbon black pigment); (4) electrical (mineral-filled with very low water absorption and improved insulation resistance); (5) heat resistant (normally mineral-filled); and (6) special (compounds developed for specific chemical resistance, etc. which may have unique combinations of properties). See "Phenolics," Modern Plastics Encyclopedia, Vol. 60, pp. 341-35. (McGraw-Hill, 1983).
Unfortunately, the standard phenolic molding compositions used for the manufacture of elcap covers permit the extraction of corrosion causing halides (principally chlorides) from the inner surfaces of the cover. This halide extraction characteristic of standard covers, especially in combination with the low corrosion inhibition capability of high performance electrolytes, increases the susceptibility of the elcap to corrosion and precludes the construction of reliable, long life elcaps which can be used at higher temperatures.
Other commercially available cover materials, such as Nylon.RTM., prevent to a greater extent the extraction of halides but are flammable, mechanically weak, and subject to melting, cracking, crazing and other forms of deterioration. Some expensive specialty polymers exist which may be suitable for the manufacture of elcap covers, but they are not practical for commercial use due to their low availability and/or high cost.
Accordingly, a clear need exists for improved molding compositions useful for the manufacture of elcap covers that are economical to produce, that can withstand high temperatures (even when used with high performance electrolytes with their lower corrosion inhibiting properties), and that offer long operating life and mechanical strength.
It is an object of the present application to provide a process for producing elcap covers that inhibits or eliminates the extraction of halides, particularly chlorides, from the cover during operation and which process can use commercially available molding compositions. It is also an object of the present invention to provide improved elcap covers which exhibit lower extraction of halides during operation. Further, it is an object of the present invention to provide elcap covers that have low halide extractability characteristics while maintaining suitable mechanical strength, low flammability and long service life. Finally, it is also an object of the invention to provide covers which are suitable for use in electrolytic capacitors at higher temperature and with high performance electrolytes.
These and other objects and advantages will become apparent to those skilled in the art upon a reading of the present disclosure.